The classification scheme for fine-grained sedimentary rocks: A review and a new approach based on five inherent rock attributes

Abstract

Despite global stratigraphic and economic significance, uncertainty remains concerning the general lithofacies classification and nomenclature of the fine-grained sedimentary rocks (FGSR). This study reviewed existing nomenclatures of FGSR and discussed their potential issues. Currently, none of the existing nomenclatures of FGSR can simultaneously: 1) capture the variations in physical, chemical, and biological characteristics of FGSR at micron-to-meter scales (i.e., serve as a “description” function); 2) facilitate the assessment of the primary deposition process and mechanism (i.e., serve as an “assessment” function); and 3) predict post-depositional diagenetic pathways and evolution of bulk physical properties (i.e., serve as a “prediction” function).

To have a classification scheme that balances the above three functions, an inclusive nomenclature is proposed in this review based on five inherent rock attributes: textural variation (grain size), mineral composition and grain origin, bedding style, total organic carbon (TOC) content, and organic matter (OM) type. This review suggests describing FGSR by a root term based on these five inherent rock attributes because they are independent of each other and recorded complete variations of physical, chemical, and biological characteristics of FGSR, which can enhance our understanding of the primary deposition process and post-depositional diagenesis that creates the enormous heterogeneity of bulk physical properties of rocks.

Application of this nomenclature to two FGSR systems with different geological contexts, including the Pennsylvanian Cline Shale, marine foreland Midland Basin, and the Jurassic Dongyuemiao Member, lacustrine Sichuan Depression, illustrates the immense heterogeneities commonly observed in FGSR and demonstrates how detailed characterization in micron-to-meter scales physical, chemical, and biological variation of FGSR can be used to assess the primary deposition process and predict evolutions of bulk physical properties. This inclusive nomenclature provides comparable and consistent terminologies across the entire spectrum of FGSR, and thus, all types of FGSR can be consistently described and correlated.